JBL system and upgraded speakers

SO I just got a box and baffles from marv and have 2 pairs of 6 1/2 polk speakers coming from crutchfield. I thought the JBL system came with an amp installed under the drivers seat or someplace for the front door speakers. Can I use that amp to run the new door speakers or is it only good for JBL package....It is in a 2009 double cab.

My plan was to run a small amp to power the front door speakers (and tweater) and run the rear door speakers from the HU (kenwood 8120)

One shop guy told me that the JBL system is sort of all or nothing. meaning that if you replace anything you have to replace everything. I thought it was his "sales pitch" but I know some audio systems do work like that.